[firedrake] Problem with Jacobian

[Buesing, Henrik]

> -----Ursprüngliche Nachricht-----
> Von: firedrake-bounces at imperial.ac.uk [mailto:firedrake-
> bounces at imperial.ac.uk] Im Auftrag von Lawrence Mitchell
> Gesendet: 05 November 2015 11:08
> An: firedrake at imperial.ac.uk
> Betreff: Re: [firedrake] Problem with Jacobian
> 
> 
> > On 5 Nov 2015, at 09:49, Buesing, Henrik <HBuesing at eonerc.rwth-
> aachen.de> wrote:
> >
> > Dear all,
> >
> > I’m having a variable Sw, which I calculate pointwise in a routine
> calc_Sw (see attachment). This variable depends on my primary unknown h:
> Sw = (h-hn)/(hw-hn) (hn,hw known values).
> >
> > But now it seems like automatic differentiation for this routine does
> not work. I’m getting zero entries for the Jacobian, whereas d(Sw)/dh =
> 1.0 should hold.
> 
> Yes, this is because the AD doesn't know about the relationship between
> Sw and dh.  UFL has a facility for this, but I notice we don't expose it
> in firedrake (however, it is straightforward to do):
> 
> 
> We want
> 
> derivative(F, u)
> 
> But F contains a coefficient, S, whose derivative wrt u is 1.0 (however,
> they are not symbolically related in a way UFL understands).  So we
> build a mapping from this coefficient to its derivative wrt u:
> 
> coefficient_derivatives = {S: 1.0}
> 
> and then pass this additional information to the derivative call.
> 
> derivative(F, u, coefficient_derivatives=coefficient_derivatives)
> 
> Firedrake uses the UFL derivative function, but does not expose this
> extra argument in the interface.  It is straightforward to alter the
> definition in firedrake/ufl_expr.py to take this extra argument and pass
> it through. 
[Buesing, Henrik] So I just add 

def derivative(form, u, du=None,coefficient_derivatives=None):
""
return ufl.derivative(form, u, du, coefficient_derivatives)

or how would this work? And then I have to reinstall firedrake?


> If this works for you, do you want to propose a patch that
> adds this functionality?
> 
[Buesing, Henrik] How would I do this?

> Cheers,
> 
> Lawrence